Optical sensors, such as optical proximity sensors, typically include a light source and an adjacent photosensitive light detector. Such optical sensors can be used to detect the presence of an object, estimate proximity of an object and/or detect motion of an object, based on the magnitude of light originating from the light source that is reflected from an object and detected by the light detector. The value of these sensors has become more important with the advent of battery-operated handheld devices, such as mobile phones. For example, a fair amount of the energy from a mobile phone battery is used to drive the display, and there is value in turning off the display or backlight when the mobile phone or other device is brought to the user's ear (where it cannot be viewed anyway). Optical proximity sensors have been used for this, and many other applications.
For other examples, there are many other applications in which the presence of an object can be detected with an optical proximity sensor to advantage. These range from sensing when protective covers have been opened on machinery, paper has been positioned correctly in a printer, or an operator's hands are at risk near an operating machine. An optical proximity sensor can also be used as a simple touch or near-touch activated switch, and could be implemented in applications like keyboards or devices that have a plastic housing that is sealed but which allows the light from the source to pass through and be sensed by the detector on the return.
Light from the source to the detector that is not transmitted toward the target object, but rather is transmitted directly from the source to the detector, reduces the capability of the overall device to sense distance. Such light essentially propagates sideways within the package and is considered noise or “light leakage”, and contains no information. To reduce and preferably prevent light leakage, an opaque light barrier is typically used to isolate the light source from the light detector. However, light barriers increase the complexity, cost and size of optical sensors. Additionally, light barriers are often imperfect, resulting in light leaking under, over and/or through the barrier.
Optical sensors are often used with (e.g., placed behind and/or covered by) a cover plate that is glass, plastic, or some other protective light transmissive material. For example, the cover plate can be the glass covering a screen of a mobile phone, portable music player or personal data assistant (PDA), or the plastic covering a screen of a laptop, netbook or tablet computer. When such a cover plate is placed over an optical sensor, the optical sensor is often susceptible to specular reflections. Specular reflections similarly reduce the capability of the overall device to sense proximity, since specular reflections are essentially noise that contain no information.
In view of the above, there has been a desire to minimize light being transmitted directly from a light source to a light detector, as well as to minimize specular reflections and/or other internally reflected light. Conventional attempts to achieve these goals typically relate to modification of the mechanical/structural design of optical sensors.